Theoretical Study of the Hydroxyl-Radical-Initiated Degradation Mechanism, Kinetics, and Subsequent Evolution of Methyl and Ethyl Iodides in the Atmosphere

The degradation and transformation of iodinated alkanes are crucial in the iodine chemical cycle in the marine boundary layer. In this study, MP2 and CCSD(T) methods were adopted to study the atmospheric transformation mechanism and degradation kinetic properties of CH3I and CH3CH2I mediated by (OH)-O-center dot radical. The results show that there are three reaction mechanisms including H-abstraction, I-substitution and I-abstraction. The H-abstraction channel producing (CH2I)-C-center dot and CH3C center dot HI radicals are the main degradation pathways of CH3I and CH3CH2I, respectively. By means of the variational transition state theory and small curvature tunnel correction method, the rate constants and branching ratios of each reaction are calculated in the temperature range of 200-600 K. The results show that the tunneling effect contributes more to the reaction at low temperatures. Theoretical reaction rate constants of CH3I and CH3CH2I with (OH)-O-center dot are calculated to be 1.42x10(-13) and 4.44x10(-13) cm(3) molecule(-1) s(-1) at T=298 K, respectively, which are in good agreement with the experimental values. The atmospheric lifetimes of CH3I and CH3CH2I are evaluated to be 81.51 and 26.07 day, respectively. The subsequent evolution mechanism of (CH2I)-C-center dot and CH3C center dot HI in the presence of O-2, NO and HO2 indicates that HCHO, CH3CHO, and I-atom are the main transformation end-products. This study provides a theoretical basis for insight into the diurnal conversion and environmental implications of iodinated alkanes.

Automated summary

This study investigated the atmospheric transformation mechanism and degradation kinetics of CH3I and CH3CH2I mediated by (OH)-O-center dot radical using MP2 and CCSD(T) methods. The results revealed three reaction mechanisms and identified H-abstraction channels as the main degradation pathways for CH3I and CH3CH2I. The rate constants and branching ratios of each reaction were calculated, showing the contribution of tunneling effect at low temperatures. The theoretical rate constants agreed well with experimental values, and the atmospheric lifetimes of CH3I and CH3CH2I were evaluated to be 81.51 and 26.07 days. The subsequent evolution mechanism indicated the formation of HCHO, CH3CHO, and I-atom as the main transformation end-products. This study provides insights into the diurnal conversion and environmental implications of iodinated alkanes.